1. Field of the Invention
The present invention relates to a modulator for generating a modulation signal for amplitude-modulating a radio-frequency signal, and particularly to a modulator having a function of correcting nonlinearity, and also a transmitter and a communication terminal, each using the modulator.
2. Description of the Background Art
An example of a system for amplitude-modulating a radio-frequency signal may include a polar modulation system. FIG. 16 is a diagram showing an example structure of a polar modulator 900 used in a transmitter of a conventional polar modulation system (see FIG. 1 of “Simple polar-loop transmitter for dual-mode Bluetooth”, Takashi Oshima and Masaru Kokubo, Hitachi Ltd., Central Research Laboratory, Circuits and Systems, 2005. ISCAS 2005. IEEE International Symposium on 23-26 May 2005 Page(s): 3966-3969 Vol. 4).
In FIG. 16, the conventional polar modulator 900 includes an I/Q conversion section 901, a polar conversion section 902, an AM modulation section 903, a PM modulation section 904, and a power amplifier (PA) 905. The I/Q conversion section 901 receives transmission data including information to be transmitted. The I/Q conversion section 901 converts the transmission data into an in-phase component and a quadrature component, and outputs the in-phase component and the quadrature component as an I signal and a Q signal which are digital, respectively. The polar conversion section 902 converts the I signal and the Q signal which are outputted from the I/Q conversion section 901, into an amplitude component and a phase component of polar coordinates, and outputs the amplitude component and the phase component as an amplitude signal R and a phase signal θ which are digital, respectively. The AM modulation section 903 converts the amplitude signal R into an analog value, and inputs the analog value to the power amplifier 905. The PM modulation section 904, which includes a PLL circuit containing a voltage controlled oscillator, changes the phase of an output signal from the voltage controlled oscillator based on the phase signal θ, and inputs a phase-modulated radio-frequency signal to the power amplifier 905. The power amplifier 905 modulates the amplitude of the radio-frequency signal outputted from the PM modulation section 904, based on the amplitude signal R outputted from the AM modulation section 903, and outputs the modulated signal to an antenna side. As a result, the transmission data is polar-modulated and then transmitted from an antenna.
In the conventional polar modulator 900, distortion may occur in the amplitude signal R due to nonlinearity of the AM modulation section 903 and the power amplifier 905. Therefore, a system is proposed for correcting such nonlinearity. FIG. 17A is a diagram showing an example structure of the AM modulation section 903 having a conventional system for correcting the nonlinearity. In FIG. 17A, the AM modulation section 903 includes a table section 906, a correction section 907, a digital-to-analog conversion section 908, a first amplification section 909, and a second amplification section 910.
FIG. 17B is a diagram showing the relationship between an input digital value to the AM modulation section 903 and an output analog value from the power amplifier 905. As shown in FIG. 17B, the input digital value is in a nonlinear relationship with the output analog value. Particularly, in the relationship between the input digital value and the output analog value, an offset value a is produced due to the offset of the digital-to-analog conversion section 908 and the DC offset voltages of the first amplification section 909, the second amplification section 910 and the power amplifier 905. In order to correct the nonlinearity and the offset value a, the AM modulation section 903 corrects the amplitude signal R in the table section 906 and the correction section 907.
FIG. 17C is a diagram showing the characteristics of a correction table stored in the table section 906. In FIG. 17C, a dashed curve is input/output characteristics shown in FIG. 17B and a solid curve is the characteristics of the correction table. The correction table is created based on the reverse characteristics of the input/output characteristics shown in FIG. 17B. In the correction table, the offset value a is added to the input digital value such that when the input digital value is “0”, the corrected digital value is “a”.
The correction section 907 outputs, based on the table section 906, a digital signal having the corrected digital value corresponding to the digital value of the inputted amplitude signal R. The digital-to-analog conversion section 908 converts the corrected digital signal into an analog signal, and inputs the analog signal to the first amplification section 909. The first amplification section 909 amplifies the analog signal from the digital-to-analog conversion section 908, in accordance with transmission power requested by the system. The second amplification section 910 amplifies by a certain fixed gain the analog signal outputted from the first amplification section 909, and inputs the analog signal to the power amplifier 905, such that the power amplifier 905 can operate. The power amplifier 905 amplitude-modulates a phase signal which is a radio-frequency signal, based on the analog amplitude signal inputted from the second amplification section 910. As a result of the corrections made by the correction section 907, the input/output characteristics are linearized as indicated by a solid straight line in FIG. 17D.
As shown in FIG. 17C, the conventional AM modulation section 903 prepares the correction table based on the reverse characteristics of nonlinearity characteristics, and corrects the input digital signal. However, the characteristics of the devices used in the AM modulation section 903 temporally change due to so-called aging characteristics, temperature changes, and the like. Accordingly, the nonlinearity characteristics also temporally change. Particularly, the temporal changes of the offset value may be great depending on a device. The conventional AM modulation section 903, however, cannot correct the input digital signal in accordance with the above-described temporal changes of the offset value.